Interferometric measurements using phase-modulation or optical heterodyne techniques are presently capable of measurement resolutions of one nanometer. Because of phase ambiguities, these are typically relative measurements of displacement from an initial position with a limit on the allowable displacement between measurements of .+-..lambda./4, where .lambda. is the source wavelength. For near-infrared sources, such as GaAlAs laser diodes, .lambda./4 is on the order of 200 nm. However, this ambiguity interval is sufficiently small that air turbulence vibration or momentary interruptions of the beam may adversely affect phase tracking techniques for displacement measurement. Furthermore, if the measurement bandwidth is low, phase tracking may not be possible and the dynamic range is reduced to the .+-..lambda./4 ambiguity interval.
In order to maintain interferometric accuracy while eliminating phase ambiguities over a larger range of displacements, two wavelengths can be employed to generate a synthetic wavelength .LAMBDA. that is longer than either of the optical wavelengths .lambda..sub.1 or .lambda..sub.2. With such a two color source, interferometric metrology can be performed within the significantly longer, range of .+-..LAMBDA./4, without ambiguity and without sacrificing the accuracy of the single-source instrument.
The following journal articles all discuss aspects of employing two source wavelengths for interferometric metrology: K. Creath, "Step Height Measurement Using Two-Wavelength Phase-Shifting Interferometry", Appl. Opt. 26(14), 2810 (1987). J. C. Wyant, "Testing Aspherics Using Two-wavelength Holography", Appl. Opt. 10(14) 2113-2118 (1971); C. Polhemus, "Two-wavelength Interferometry", Appl. Opt. 12(14) 2071-2074 (1973); Y. Cheng and J. C. Wyant, "Two-wavelength Phase Shifting Interferometry", Appl. Opt. 23(29), 4539-4543 (1984); and K. Creath, Y. Cheng and J. C. Wyant, "Contouring Aspheric Surfaces Using Two-wavelength Phase-shifting interferometry", Opt. Act. 32 (17), 1455-1464.
Although the usefulness of synthetic wavelengths for removing phase ambiguities has been well documented, the practical application of these techniques depends on the availability of compact, efficient and reliable multiple-wavelength sources. One approach is to use two single-wavelength, GaAlAs laser diodes, which are commercially available with a variety of far-red and near-infrared wavelengths.
The following journal articles discuss the application of two laser diodes: C. C. Williams and H. K. Wickramasinghe, "Absolute Optical Ranging With 200 nm Resolution:, Opt. Let. 14(11), 542-544 (1989); C. C. Williams and H. K. Wickramasinghe, "Optical Ranging by Wavelength Multiplexed Interferometry" J. Appl Phys. 60(6), 1900-1903 (1986); A. J. den Boef, "Two-wavelength Scanning Spot Interometer using Single-frequency Diode Lasers", Appl. Opt. 27 (2) 306-311 (1988); and A. F. Fercher, U. Vry and W. Werner, "Two-wavelength Speckle Interferometry on Rough Surfaces Using a Mode Hopping Diode Laser", Optics and Lasers in Engineering 11, 271-279 (1989).
One significant advantage of laser diodes for two-color interferometry is the wavelength-tunability of these devices with temperature. However, this advantage also presents a problem of long-term wavelength drift due to thermal effects. This drift renders the synthetic wavelength data useless for the purpose of resolving phase ambiguities. The latter group of journal articles referenced above apply principally to imaging applications involving rapid scanning and small optical path differences, and the importance of wavelength stabilization is not addressed. For applications demanding long-term repeatability with optical path differences of greater than a millimeter, the requirements for wavelength control as well as methods for achieving the required control must be considered.
Before describing in detail the presently preferred embodiment of the invention a discussion is first made of the wavelength stability requirements of a conventional one-color, or wavelength, interferometer and a two-color interferometer.
The terms optical separation, fringe number, fringe order and uncertainty are introduced and defined for use in the subsequent consideration of the two color system of the invention.